Future radial velocity, astrometric, and direct-imaging surveys will find nearby Earth-sized planets within the habitable zone in the near future. How can we search for water and oxygen in those nontransiting planets? We show that a combination of hi
gh-dispersion spectroscopic and coronagraphic techniques is a promising technique to detect molecular lines imprinted in the scattered light of Earth-like planets (ELPs). In this method, the planetary signals are spectroscopically separated from telluric absorption by using the Doppler shift. Assuming a long observing campaign (a 10-day exposure) using a high-dispersion spectrometer (R=50,000) with speckle suppression on a 30-m telescope, we simulate the spectra from ELPs around M dwarfs (whose stellar effective temperature is 2750-3750 K) at 5 pc. Performing a cross-correlation analysis with the spectral template of the molecular lines, we find that raw contrasts of $10^{-4}$ and $10^{-5}$ (using Y, J, and H bands) are required to detect water vapor at the 3 $sigma$ and 16 $sigma$ levels, respectively, for $T_star$=3000 K. The raw contrast of $10^{-5}$ is required for a 6 $sigma$ detection of the oxygen 1.27 $mu$m band. We also examine possible systematics, incomplete speckle subtraction, and the correction for telluric lines. When those are not perfect, a telluric water signal appears in the cross-correlation function. However, we find the planetary signal is separated from that resulting from the velocity difference. We also find that the intrinsic water lines in the Phoenix spectra are too weak to affect the results for water detection. We conclude that a combination of high-dispersion spectroscopy and high-contrast instruments can be a powerful means to characterize ELPs in the extremely large telescope era.
We present a test for spin-orbit alignment for the host stars of 25 candidate planetary systems detected by the {it Kepler} spacecraft. The inclination angle of each stars rotation axis was estimated from its rotation period, rotational line broadeni
ng, and radius. The rotation periods were determined using the {it Kepler} photometric time series. The rotational line broadening was determined from high-resolution optical spectra with Subaru/HDS. Those same spectra were used to determine the stars photospheric parameters (effective temperature, surface gravity, metallicity) which were then interpreted with stellar-evolutionary models to determine stellar radii. We combine the new sample with the 7 stars from our previous work on this subject, finding that the stars show a statistical tendency to have inclinations near 90$^circ$, in alignment with the planetary orbits. Possible spin-orbit misalignments are seen in several systems, including three multiple-planet systems (KOI-304, 988, 2261). Ideally these systems should be scrutinized with complementary techniques---such as the Rossiter-McLaughlin effect, starspot-crossing anomalies or asteroseismology---but the measurements will be difficult owing to the relatively faint apparent magnitudes and small transit signals in these systems.
We report a discovery of a companion candidate around one of {it Kepler} Objects of Interest (KOIs), KOI-94, and results of our quantitative investigation of the possibility that planetary candidates around KOI-94 are false positives. KOI-94 has a pl
anetary system in which four planetary detections have been reported by {it Kepler}, suggesting that this system is intriguing to study the dynamical evolutions of planets. However, while two of those detections (KOI-94.01 and 03) have been made robust by previous observations, the others (KOI-94.02 and 04) are marginal detections, for which future confirmations with various techniques are required. We have conducted high-contrast direct imaging observations with Subaru/HiCIAO in $H$ band and detected a faint object located at a separation of $sim0.6$ from KOI-94. The object has a contrast of $sim 1times 10^{-3}$ in $H$ band, and corresponds to an M type star on the assumption that the object is at the same distance of KOI-94. Based on our analysis, KOI-94.02 is likely to be a real planet because of its transit depth, while KOI-94.04 can be a false positive due to the companion candidate. The success in detecting the companion candidate suggests that high-contrast direct imaging observations are important keys to examine false positives of KOIs. On the other hand, our transit light curve reanalyses lead to a better period estimate of KOI-94.04 than that on the KOI catalogue and show that the planetary candidate has the same limb darkening parameter value as the other planetary candidates in the KOI-94 system, suggesting that KOI-94.04 is also a real planet in the system.
We present precise radial velocities of XO-2 taken with the Subaru HDS, covering two transits of XO-2b with an interval of nearly two years. The data suggest that the orbital eccentricity of XO-2b is consistent with zero within 2$sigma$ ($e=0.045pm0.
024$) and the orbit of XO-2b is prograde (the sky-projected spin-orbit alignment angle $lambda=10^{circ}pm72^{circ}$). The poor constraint of $lambda$ is due to a small impact parameter (the orbital inclination of XO-2b is almost 90$^{circ}$). The data also provide an improved estimate of the mass of XO-2b as $0.62pm0.02$ $M_{rm Jup}$. We also find a long-term radial velocity variation in this system. Further radial velocity measurements are necessary to specify the cause of this additional variation.
We report photometric and radial velocity observations of the XO-4 transiting planetary system, conducted with the FLWO 1.2m telescope and the 8.2m Subaru Telescope. Based on the new light curves, the refined transit ephemeris of XO-4b is $P = 4.1250
828 pm 0.0000040$ days and $T_c [BJD_TDB] = 2454485.93323 pm 0.00039$. We measured the Rossiter-McLaughlin effect of XO-4b and estimated the sky-projected angle between the stellar spin axis and the planetary orbital axis to be $lambda = -46.7^{circ} ^{+8.1^{circ}}_{-6.1^{circ}}$. This measurement of $lambda$ is less robust than in some other cases because the impact parameter of the transit is small, causing a strong degeneracy between $lambda$ and the projected stellar rotational velocity. Nevertheless, our finding of a spin-orbit misalignment suggests that the migration process for XO-4b involved few-body dynamics rather than interaction with a gaseous disk. In addition, our result conforms with the pattern reported by Winn et al. (2010, ApJL, 718, L145) that high obliquities are preferentially found for stars with effective temperatures hotter than 6250~K.
